The object of this study is to develop an in vitro potency assay to replace the guinea pig death test portion of the official release test for the diphtheria component of adsorbed toxoid vaccines. VERO cells are being used as a target for the diphtheria toxin. Collaborative studies were conducted using slightly different methods to detect antibody neutralization of diphtheria toxin as determined by inhibition of cell death. Coded samples distributed to the four facilities showed good agreement in results, despite variations in cell death detection methods. A pool of guinea pig sera has been prepared by one of the collaborators to serve as a secondary standard; testing has begun to compare this standard with the existing horse serum antitoxin standard. Based on preliminary evaluation of the two standards in concurrent animal and cell tests, the lower toxin dose assay is being included to determine whether the additional sensitivity of this test can contribute to the overall evaluation of products for release. The LRR plans to prepare the guinea pig standard in a freeze-dried form to concurrently evaluate the new freeze-dried horse serum antitoxin preparation in high and low dose toxin tests in multiple laboratories.